The present invention relates to a formable polyimide film molded article used as a reflector substrate for an illumination device. The reflector substrate has deposited onto it a reflecting surface to form a reflector. The reflector is used as a lamp reflector mainly in automotive headlights or fog lights.
Lamp reflectors generally used in automotive headlights and fog lights are composed of reflector substrates made of glass, metal (e.g., aluminum), or injection or compression-molded plastic (e.g., polyphenylene sulfide, unsaturated polyester resin) on to which a metal reflective layer is deposited using aluminum, nickel-chromium alloy, titanium oxide or the like as the reflecting layer.
Because light sources such as halogen bulbs and xenon bulbs are primarily used in automotive headlights and fog lights for high luminescence, the temperature of the fixture near the base of the bulb rises to over 200xc2x0 C. For this reason, current reflector substrates are made of glass or metal which are heavy, or are made as an injection or compression molded plastic article containing an additional inorganic reinforcement layer such as glass fibers or calcium carbonate (i.e., a reinforced resin molding) which is currently adhered to the substrate by subsequent processing steps.
The use of glass as the reflector substrate presents a number of problems, such as its heavy weight and the ease with which it breaks if placed under stress. When a metal or a reinforced resin molding is used as the reflector substrate, the heavy weight is compounded by other problems, primarily the need to carry out secondary processing steps, such as resin coating prior to vapor deposition, to the reflective substance. The result is an increased number of manufacturing steps to form the part.
Reinforced resin moldings provide a lightweight alternative to metal reflector substrates. However, these resin moldings must have considerable wall thickness both to provide the substrate with the necessary strength and because of inherent limitations in the actual molding technique. One adverse result is the low heat dissipation of the substrate itself. In lamps for example, where the construction is prone to heat buildup (e.g., projector type lamps), the thermal stability of the resin molding is not entirely adequate for good shape retention in the presence of heat. Moreover, the thick wall of the substrate (an insulating material) promotes a temperature rise within the housing, which in turn, shortens the life of the bulb.
Another problem encountered with the use of a reinforced resin moldings as the reflector substrate is that the material can generate gases such as chlorine compounds when the unit is lit and upon heating of the lamp. These gases will condense and deposit, for example, on the front lens of an automotive headlight. This can form a mist, or condensate, on the inside of the lens housing which of course impedes the performance of the lamp.
Previous art describes a sheet-type reflector made of a film sheet that has been mirror-treated by the vapor deposition of aluminum, and discloses that this sheet-type reflector does not require pretreatment such as degreasing to achieve the desired properties. However, the film sheet used in this case is made of polyethylene, which has a poor heat resistance. Accordingly, a polyethylene sheet-type reflector cannot be used as a reflector substrate in headlamps that employ lights which generate a high amount of heat, such as halogen bulbs or xenon bulbs, due to the film sheet""s inadequate shape retention upon heat exposure.
Other art discloses a reflecting mirror comprising a visible light-reflecting infrared light-transmitting multi-layer film on a reflector base made of a polyimide resin and polyetherketone resin film. In addition to having a high resistance to breakage, this reflecting mirror is also described as capable of withstanding heat generation from halogen bulbs and the like because it is made of polyimide resin. Yet, given that this reflecting mirror is composed of a visible light-reflecting, infrared light-transmitting multi-layer film formed on the substrate, it cannot be put to practical use as a reflector in automotive headlights and fog lights.
The present invention was ultimately arrived at following investigations aimed at resolving the above-described problems in the related art.
It is therefore the first goal of the invention to provide a reflector substrate formable into a main body for use as an illumination device which, is lightweight, has excellent surface smoothness, is safe, is easily processable, is heat resistance, and is easy to handle.
A second goal of the invention is to provide a low-cost, high-quality lamp reflector which is suitable for large-volume production because it does not require pre-treatment of the reflector substrate at the time the reflecting layer is formed.
To achieve the first goal, the main body of the illumination device of the present invention is comprised of a shaped article made of a formable thermoplastic aromatic polyimide film having excellent heat resistance. Accordingly, it has a good surface smoothness and does not require pre-treatment at the time of reflective layer formation. Moreover, it is the use of a thermoplastic aromatic polyimide formulation having a glass transition temperature of 210 to 290xc2x0 C. which enables the main body of the illumination device to be produced rapidly and at low cost by a vacuum forming process. In addition, it is lightweight and resistant to breakage which makes it an excellent, safety, and handleable unit.
To achieve the second goal the illumination device of the invention is comprised of a main body composed of a shaped article made of a specific plastic film having excellent heat resistance and surface smoothness on which a reflective substrate has been formed by vapor deposition. Accordingly, there is no need for pre-treatment such as resin coating prior to vapor deposition of the reflective substrate, which enables the lamp reflector to be mass-produced at low cost. Moreover, the illumination device does not degas which becomes a practical obstacle when other devices are lit. Hence, in addition to polyimide""s excellent heat dissipation and safety, the reflector substrate exhibits other high quality performance features such as high heat resistance and excellent specular properties. This combination of features make it advantageous for use as an illumination device in automotive headlights and fog lights, and also large floodlights.